1. Field of the Invention
The present invention relates to a patterning device for use with optical projection lithography comprising a substrate and a pattern on a surface of the substrate, the pattern including a plurality of dies.
2. Description of the Prior Art
In a conventional lithographic method a lithographic apparatus is used to image a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of semiconductor devices including integrated circuits (IC devices). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC device. On the reticle, the circuit patterns are arranged in one or more dies, each die corresponding to a layer of the semiconductor device. Usually, when the mask pattern includes a plurality of dies, each die corresponds to the same layer. This pattern of dies can be transferred onto a target portion on a substrate (e.g. a silicon wafer).
The lithographic apparatus comprises an illumination system to illuminate the mask and a projection system (also referred to as a projection lens) to transfer the pattern, via imaging, onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Known lithographic apparatus include steppers or step-and-repeat apparatus, and scanners or step-and-scan apparatus. In a stepper each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and the wafer is moved by a predetermined amount to a next position for a next exposure. In a scanner each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction, and next the wafer is moved to a next position for a next exposure.
Conventional optical projection lithography apparatus are used for exposing a resist layer on a substrate to a demagnified image of a mask pattern. The mask pattern is illuminated by radiation having a wavelength of, for example, 365 nm, or 248 nm, or 193 nm. Common materials for the lens elements of projection systems for use with radiation of 248 nm or 193 nm wavelength are quartz and CaF2. These optical materials have a sufficiently high transmission for 248 and 193 nm radiation. However, in use some radiation is absorbed by these materials. Also, anti-reflection coatings on the surfaces of the optical lens elements may have a residual absorbance for the radiation used for imaging. Such absorbance causes a transfer of heat to the lens elements, and a subsequent thermal deformation of the lens elements.
Any such thermal deformation may lead to image aberration beyond tolerance, occurring during operation of the lithographic apparatus. Therefore, in optical projection lithography there is a need to control the image aberration (also referred to as optical aberration) due to thermal deformation of lens elements of the projection system.
An optical aberration can be thought of as consisting of constituent aberration contributions such as, for example, the commonly known lower-order aberrations called spherical aberration, coma, astigmatism, image curvature and distortion. Higher-order aberrations of these and other types are generally present as well in the optical aberration. Any of the aberration contributions can be categorized as either a symmetric aberration or an asymmetric aberration. A symmetry of an aberration contribution may relate to either a symmetry of the aberration magnitude with respect to the optical axis of the corresponding optical system, or to a symmetry of the aberration magnitude with respect to the center of an object field of the optical system. The object field of a projection lens of a projection lithography apparatus corresponds to the maximum area of a reticle which can be used for transfer of an IC layer pattern to a target portion on the substrate by imaging, using the projection system. This useable maximum area of a reticle is referred to hereinafter as the “field”. An aberration such as for example lower and higher order spherical aberration which is rotationally symmetric with respect to the optical axis of the projection lens, may yet be asymmetrically distributed over the field, and be classified as an asymmetric aberration or asymmetric field distributed aberration.
Conventionally, control of image aberrations is achieved by position adjustment of one or more lens elements of the projection system, or by an adjustment of the mask and substrate positions and orientations, or by a combination of any of these adjustments. In particular, an adjustment of projection lens elements along an optical axis of the projection system (referred to as an adjustment in the z-direction or a z-adjustment) is suitable for correcting symmetric optical aberrations, such as may be caused by a symmetric thermal deformation of one or more lens elements of the projection system.
The correction of asymmetric aberrations (for example due to asymmetric thermal deformations of one or more lens elements) is much more difficult and generally only partially possible. For their reduction adjustment of lens element positions can still be used, however, lateral adjustments perpendicular to the direction of axial z-adjustments (referred to as x,y-adjustments) are generally required. To provide x,y-adjustments to one or more lens elements, dedicated lens manipulators must be incorporated in the projection system. Also, manipulators to provide a tilt to one or more lens elements may be needed. Since there is only a limited amount of any such actuators available, there is the problem of avoiding any inducement of asymmetric aberrations, in use, where these aberrations are absent or within tolerance with the optical projection system before use.